1. Field of the Invention
The present invention relates to a turbocharger control system for supplying pressurized air to an internal combustion engine, and more particularly, to a turbocharger control system having a rotary electric machine mounted on a rotatable shaft by which a turbine and a compressor are interconnected.
2. Description of the Related Art
The internal combustion engines on many motor vehicles are provided with turbochargers. Immediately after the accelerator pedal of such a motor vehicle is depressed to start the motor vehicle, since the rotational speed of the engine is low, the energy of exhaust gases discharged from the engine is low. Therefore, the rotational speed of the engine is also low, and so is the boost pressure produced by the turbocharger. As the accelerator pedal is continuously depressed, the engine rotational speed is progressively increased, and so is the exhaust energy. Consequently, the rotational speed of the turbocharger is increased and the boost pressure is also increased.
As described above, an internal combustion engine equipped with a turbocharger suffers a time lag or delay after the accelerator pedal is depressed until the boost pressure produced by the turbocharger is actually increased. Such a time lag is referred to as a "turbo lag", and is responsible for the poor acceleration of a motor vehicle with a turbocharged engine when the motor vehicle is started.
To reduce such a turbo lag, there has been proposed in Japanese Laid-Open Patent Publication No. 60(1985)-195329 a turbocharger control system in which a rotary electric machine is mounted on the rotatable shaft of a turbocharger combined with an engine on a motor vehicle. When the accelerator pedal of the motor vehicle is depressed, the rotary electric machine is operated as an electric motor to assist in rotating the turbocharger, thereby increasing the boost pressure thereof while the engine is rotating at a low speed.
The proposed turbocharger control system greatly reduces the turbo lag at a start of the motor vehicle. However, the response of acceleration of the motor vehicle to a depression of the accelerator pedal is slightly lower than that of a motor vehicle which is equipped with a supercharger. Such a response delay is caused for the following reason: The supercharger produces a boost pressure even while the engine is idling, but the turbocharger does not produce any substantial boost pressure while the engine is idling. Even if the rotary electric machine coupled to the turbocharger is operated as an electric motor when the accelerator pedal is depressed, it takes a little time before the rotational speed of the rotary electric machine reaches a predetermined speed to assist the turbocharger.